This invention relates to the transmission of information. The invention has a particular application to the field of high speed digital wireless communications.
Various techniques are known for transmitting information using frequency modulation. The technique known as OFDM (orthogonal frequency division multiplexing) has certain advantages, particularly for wireless communications wherein impairments resulting from multipath propagation, such as in indoor environments, are experienced. An OFDM system is described, for example, in IEEE Transactions on Communications, Vol. COM-33, No. 7, July 1985, pages 665-675, L. J. Cimini: "Analysis and Simulation of a Digital Mobile Channel Using Orthogonal Frequency Division Multiplexing". In an OFDM system signals are transmitted simultaneously over a plurality of subchannels.
An OFDM system makes efficient use of bandwidth since the spectra of the individual subchannels are permitted to overlap, with the spectra of the individual subchannels being zero at the other subcarrier frequencies. Also, the subcarrier frequencies are separated by multiples of the inverse of the signalling interval so that coherent detection of a signal element in any one subchannel gives no output for a received element in any other subchannel. One advantage of an OFDM system is that modulation and demodulation can be effected using discrete Fourier transform procedures, enabling a fast Fourier transform to be utilized. It will be appreciated that an OFDM system is based on the modulation of individual subcarriers and summation of these modulated subcarriers. This enables a high data rate to be achieved, while the modulation rate per subcarrier is lower and allows more delay spread without the use of accurate equalisation.
In one form of OFDM system, signals are encoded for transmission over the OFDM subchannels. Typically, such coding involves a phase-significant component. An example is PSK (phase shift keying) coding.
When information is transmitted over radio frequencies, an RF (radio frequency) power stage is utilized. An important parameter in connection with transmission at such frequencies is the crest factor. By the crest factor of a signal is meant the ratio of peak voltage to the RMS (root mean square) voltage. For an RF system this corresponds to the ratio of peak envelope voltage to mean (RMS) voltage.
For an OFDM system using PSK, the crest factor is high since there exists the possibility that the individual amplitudes of the OFDM subchannels can all make a positive contribution if they all have the same phase. In this connection, it will be appreciated that the average power of the combined subchannels is calculated using power summation because of the independence of the individual subchannel signals.
A high crest factor has the disadvantage that the output RF power stage has high design requirements in order to ensure correct operation having regard to signal compression, clipping and other degradation. A further disadvantage is that a high crest factor may more readily lead to peak output power levels which exceed officially prescribed limits, e.g., for the United States, levels specified in FCC (Federal Communications Commission) regulations.